Pt/L-zeolite catalysts have utility as non-acidic reforming catalysts for the octane enhancement by selective reforming of light paraffins, see P. W. Tamm et al, Catalysis 1987, J. W. Ward, Ed., Elsevier Science Publishers, B. V. Amsterdam, 1988, page 335 and Buss et al, U.S. Pat. No. 4,456,527, 1984. They are also prospective catalysts for the conversion of n-hexane to benzene, see Bernard, Proc. 5th Intern. Conf. Zeolites (L. W. Rees, Ed.) Heyden, London, 1980, page 686 and Bernard et al U.S. Pat. No. 4,104,320 (1978). The catalysts described in the latter two references were based on wide pore L-zeolite loaded with Pt particles, Pt/KL-zeolite, and those described in the first two references above were similar, but part of the K.sup.+ had been exchanged by Ba.sup.+2, Pt/BaKL-zeolite. However, both the Pt/BaKL-zeolite and Pt/KL-zeolite have low sulfur tolerance according to Tamm et al supra and it has recently been demonstrated for both catalysts that sulfur promotes the migration of Pt out of the zeolite pores and the aggregation of larger particles, see Vaarkamp et al, "The Sulfur Poisoning of Pt/BaKL Catalysts", abstract D19, 12th North American Meeting of the Catalysis Society, Lexington, Ky., May 1991 and Kao et al, "Effect of Sulfur on the Performance of Pt/KL Hexane Aromatization Catalysts", abstract D20, 12th North American Meeting of The Catalysis Society, Lexington, Ky., May, 1991. Pandey et al U.S. Pat. No. 4,680,280 (1987) discloses L-zeolite catalysts which incorporate one of the desulfurization metals, molybdenum, chromium or tungsten to improve sulfur tolerance.
Bimetallic Pt--Ni catalysts using other supports, e.g. Pt--Ni/SiO.sub.2 are reported by Jentys et al, J. Phys. Chem., 96, 1324 (1992) and Raab et al, J. Catal., 122, 406 (1990) and others as discussed infra. However, silica, alumina or carbon supported catalysts as described in the prior art are of quite low dispersion compared to the catalysts of the invention as disclosed infra.
It is known in the art that the retention of Pt in the KL-zeolite pores of of monometallic Pt/KL-zeolite catalysts is essential for high activity and selectivity, and for maintenance of high activity and selectivity, see Kao et al supra, and Iglesia et al, "A Mechanistic Proposal for Alkane Dehydrocyclization Rates on Pt/L-zeolite. Inhibited Deactivation of Pt Sites within Zeolite Channels", to be published, Proc. 10th Intern. Congr. Catal. Budapest, July 1992, and that self-deactivation of Pt/SiO.sub.2 makes such catalysts inferior to Pt/KL-zeolite, see Iglesia et al supra, even though their initial activities are comparable.
While conventional reforming catalysts are bi-functional in that they utilize support acidity as well as metal dehydrogenation/hydrogenation functionality, it has been demonstrated, see Bernard supra, that Pt/L-zeolite reforming catalysts may accomplish aromatization using Pt-only functionality and the acidity can be detrimental to optimum performance. The low sulfur tolerance of Pt/L-zeolite catalyst is known, see Bernard et al supra, but recently it has been demonstrated that the effect of sulfur is not the result of simple poisoning, see Vaarkamp et al, Kao et al supra. By mechanisms which are not fully understood, sulfur promotes Pt crystal growth and movement of Pt out of the zeolite channels.
The sulfur intolerance of Pt/L-zeolite catalyst is one of the major inhibitions to the commercial use of these catalysts for petroleum reforming or for the production of benzene or other aromatics. Thus, it is of interest to find catalyst modifiers which might stabilize Pt against crystal growth. One approach is to use first row cations to anchor Pt particles in Y-zeolite using Fe.sup.2+, see Tzou et al, Appl. Catal. 20, 231 (1986) and Balse et al, Catalysis Lett., 1, 275 (1988). Also, Cr.sup.3+ has been used to anchor Rh particles in Y-zeolite, see Tzou et al, Langmuir, 2, 773 (1986).